A drilling riser is made up of an assembly of tubular elements whose length generally ranges between 15 and 25 m, assembled by connectors. The weight of the riser borne by an offshore platform can be very great, which requires suspension means of very high capacity at the surface and suitable dimensions for the main tube and the connection fittings.
So far, the auxiliary lines: kill lines, choke lines, booster lines and hydraulic lines are arranged around the main tube and they comprise insertable fittings fastened to the riser element connectors in such a way that these high-pressure lines can allow a longitudinal relative displacement between two successive line elements, without any disconnection possibility however. Owing to these elements mounted sliding into one another, the lines intended to allow high-pressure circulation of an effluent coming from the well or from the surface cannot take part in the longitudinal mechanical strength of the structure consisting of the entire riser.
Now, in the perspective of drilling at water depths that can reach 3500 m or more, the dead weight of the auxiliary lines becomes very penalizing. This phenomenon is increased by the fact that, for the same maximum working pressure, the length of these lines requires a larger inside diameter considering the necessity to limit pressure drops.
Document FR-2,891,579 aims to involve the auxiliary lines, kill lines, choke lines, booster lines or hydraulic lines, in the longitudinal mechanical strength of the riser. According to this document, the tubes that make up an auxiliary line are assembled end to end by rigid connections allowing longitudinal stresses to be transmitted between two tubes. Thus, the auxiliary line forms a rigid assembly that affords the advantage of transmitting stresses between the top and the bottom of the riser.
One difficulty in achieving the riser according to document FR-2,891,579 lies in the assembly of two riser sections T1 and T2 shown in FIG. 1. When installing a riser at sea, section T1 is assembled end to end to section T2 of the riser. To connect them, connector C1 of main tube TB, respectively fastening means C2 and C3 of each auxiliary line tube TA, have to exactly coincide with connector C1′, respectively fastening means C2′ and C3′, of the section to be connected. Now, the manufacturing tolerances of the tubes of the main line or of the auxiliary lines can be several centimeters on 15 to 25-m long tubes. Furthermore, the welds performed between the connecting means and the tubes can increase the length difference between the various tubes of a riser section. For example, in FIG. 1, connector C1′ and fastening means C2′ and C3′ are aligned in plane P′. On the other hand, connector C2 is set back by an axial distance D1 with respect to plane P of connector C1 and connector C3 protrudes by an axial distance D2 with respect to plane P. Consequently, when connecting section T1 to section T2, while fastening means C3 abuts in C3′, connector C1 is only partly inserted in connector C1′ and fastening means C2 cannot cooperate with means C2′. The offsets in the axial position of the connectors, due to the length differences of the tubes, can make connection impossible.
The present invention aims to provide at least one of the tubes that make up the auxiliary lines with adjustment means for adjusting the axial length of the tube in order to achieve connection of the tubes between two riser sections.